Communication systems that employ link adaptation seek to maximize link utilization by selecting an information rate that is best suited for the prevalent link quality. Typically, a link adaptation scheme consists of a set of modes each incorporating a different modulation and/or coding scheme (commonly referred to as Adaptive Coding and Modulation, or ACM) or some other link parameter controlling the data rate.
Link adaptation is accomplished by establishing a set of threshold values for choosing different transmission modes. These threshold values are used to determine the selection of each mode in the adaptation scheme based on some real-time performance metric. A link adaptation system operates by periodically taking a real-time performance metric for each link (e.g., a link quality metric (LQM) or channel quality indicator (CQI)), comparing this performance metric with the threshold values for the modes, and then selecting the appropriate mode and rate that will maximize throughput.
The computation of the performance metrics can be performed directly at the receiver, e.g. as a signal-to-noise ratio (SNR), or indirectly at the transmitter, e.g. as a function of ACK/NACK messages fed back from the receiver. Traditional link adaptation systems have used the SNR or RSSI (received signal strength indicator) to determine an optimal rate to use for future transmissions (see, for example, the disclosure in U.S. Pat. No. 8,611,288), whereas other systems have used the congestion state of the network to adapt the coding and modulation (see, for example, the disclosure in U.S. Patent Application No. 2011/0194419). Other link adaptation schemes involve estimating the channel state at pilot positions that are fed back to the transmitter via a noisy feedback link. Then, using a linear prediction filter, current channel states are predicted at the transmitter and appropriate transmission modes are selected for the current transmission (see, for example, “Variable-Rate Adaptive Trellis Coded QAM for Flat-Fading Channels,” V. K. N. Lau and M. D. Macleod, IEEE Transactions on Communications, Vol. 49, No. 9, pp. 1550-1560, September 2001).
In these systems, and link adaptation systems in general, rate selection is subject to feedback delay. If the feedback delay is non-negligible with respect to the time-constant of the link quality, mismatches between the transmitted rate and the supported rate will result in unreliable transmissions. That is, a rate loss will result if the current performance metrics (that are based on the delayed performance metrics) are overestimated or underestimated. Typically, link adaptation systems based on prediction, i.e. predicting the channel state or channel parameters, are unable to account for the rate loss associated with the estimation error of the prediction process. Thus, there is a need for link adaptation systems that minimize this rate loss, thereby maximizing the throughput of the communication system.